7. ISOTOPIC COMPOSITION OF CH4, CO2 SPECIES, AND SEDIMENTARY ORGANIC MATTER WITHIN SAMPLES FROM THE BLAKE RIDGE: GAS SOURCE IMPLICATIONS1

C.K. Paull,2, 3 T.D. Lorenson,4 W.S. Borowski,2, 5 W. Ussler III,2 K. Olsen, 2, 6 and N.M. Rodriguez2, 5

ABSTRACT

The isotopic characteristics of CH4 (13C values range from -101.3 to -61.1 PDB, and D values range from -256 to -136 SMOW) collected during Ocean Drilling Program (ODP) Leg 164 indicate that the CH4 was produced by microbial CO2 reduction and that there is not a significant contribution of thermogenic CH4 to the sampled sediment gas from the Blake Ridge. The isotopic values of CO2 (13C range -20.6 to +1.24 PDB) and dissolved inorganic carbon (DIC; 13C range -37.7 to +10.8 PDB) have parallel profiles with depth, but with an offset of 12.5. Distinct downhole variations in the carbon isotopic composition of CH4 and CO2 cannot be explained by closed-system fractionation where the CO2 is solely derived from the locally available sedimentary organic matter (13C -2.0 1.4 PDB) and the CH4 is derived from CO2 reduction. The observed isotopic profiles reflect the combined effects of upwards gas migration and decreased microbial activity with depth.

1Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP, Sci. Results, 164: College Station, TX (Ocean Drilling Program).
2Geology Department, University of North Carolina, Chapel Hill, NC 27599-3315, U.S.A.
3Present address: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, U.S.A. paull@mbari.org
4Pacific Marine Geology, U.S. Geological Survey, Menlo Park, CA 94025, U.S.A.
5Present address: Exxon Exploration Company, P.O. Box 4778, Houston, TX 77210-4778, U.S.A.
6Present address: Marine Sciences, State University of New York, Stony Brook, NY 11794, U.S.A.

Date of initial receipt: 30 April 1998
Date of acceptance: 12 January 1999
Ms 164SR-207

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